15Q SOME RECENT ASTRONOMICAL EVENTS. 



greater than the corresponding field in the focus of a great mirror. 

 But notwithstanding this disadvantage, reflectors have more and more 

 come into use for photographic purposes within recent years, and 

 some of the most beautiful and striking photographs of the nebulae 

 and star clusters ever made were obtained with the Crossiey reflector 

 in the lust months of his life by Professor Keeler, director of the Lick 

 Observatory at Mount Hamilton, California. Since his untimely death 

 the instrument has been continued in use and is now giving excellent 

 results. 



More recently still Mr. Ritchey, of the Yerkes Observatory, has 

 designed and prepared with his own hands a reflecting telescope of 

 slightly smaller dimensions than the Crossiey instrument, and is now 

 obtaining photographs of nebuhe, star clusters, and.other objects requir- 

 ing much light-gathering power but no great extent of field, which 

 are unexcelled for excellence. The illustration (Plate II) shows the 

 great nebula in Cygnus as photographed by Mr. Ritchey with an 

 exposure of three hours. The faintest stars shown in the original are 

 more than 10,000 times fainter than the unaided eye can see. Plate III 

 includes two drawings from photographs by Mr. Ritchey of the nebula 

 round Nova Persei taken with the same instrument. 



I have spoken of the large expense and inconvenience attending the 

 use of refracting instruments for both visual and photographic pur- 

 poses. In preparing the great Yerkes refractor of 40 inches aperture 

 no provision was made for its employment as a photographic tele- 

 scope, but very recently, owing to the great advance made by com- 

 mercial dry -plate manufacturers in the preparation of photographic 

 plates sensitive in the yellow and green portions of the spectrum — that 

 is to say, those portions where the eye is the most sensitive — it became 

 possible, if the imperfectly focused blue and violet rays of the instru- 

 ment could be cut off, to use the telescope without prejudicially long 

 exposures for photographic purposes. Mr. Ritchey has, accordingly, 

 employed a color screen close to the photographic plate, by means of 

 which these prejudicial rays are eliminated; and by the further use 

 of a most efficient following apparatus, also of Mr. Ritchey's design, 

 there has recently been taken with this telescope, originally intended 

 only for visual and spectroscopic purposes, extraordinary perfect 

 astronomical photographs (PI. IV). This marks a most important 

 advance in astronomical photography, for it thus becomes possible, 

 with a very trifling expense, to use the great visual equatorials of the 

 world with perfect success as photographic telescopes. 



Before passing from the subject of celestial photography 1 wish to 

 mention a combination of the refracting and reflecting schemes which 

 is now being employed with great success. It will be remembered 

 that one of the most celebrated features of the Paris Exposition was 

 the " Great Telescope," so-called, and that this was employed not 



